Over the last few decades, the electronics industry has undergone a revolution by the use of semiconductor technology to fabricate small, highly integrated electronic devices. The most common semiconductor technology presently used is silicon-based. A large variety of semiconductor devices have been manufactured having various applications in numerous disciplines. Such silicon-based semiconductor devices often include metal-oxide-semiconductor (MOS) transistors, complimentary MOS (CMOS) transistors, bipolar transistors, BiCMOS transistors, etc.
Each of these semiconductor devices generally include a semiconductor substrate on which a number of active devices are formed. The particular structure of a given active device can vary between device types. For example, in MOS transistors, an active device generally includes source and drain regions and a gate electrode which modulates current between the source and drain regions. In bipolar transistors, an active device generally includes a base, a collector, and an emitter.
One important step in the formation of semiconductors is the process of electrically isolating adjacent active devices. One known technique for isolating active devices on a semiconductor substrate is LOCOS (for LOCal Oxidation of Silicon) isolation. LOCOS isolation generally involves the formation of a recessed or semi-recessed oxide in the nonactive (or isolation) areas of the substrate which separate the active devices. One alternative to LOCOS isolation is trench isolation. Trench isolation generally involves etching shallow trenches in isolation regions of the substrates and refilling the trenches with a deposited silicon dioxide layer.
In both LOCOS and trench isolation processes, an implant is typically performed to provide a channel stop doping layer beneath the isolation oxide region. In NMOS device regions, a p-type dopant such as boron is typically used, while in PMOS device regions (and in n-wells of CMOS devices) an n-type dopant such as arsenic is typically used. The use of a channel stop generally improves device performance, but nevertheless is associated with some problems. For example, when boron is used to form a channel stop, the boron readily diffuses into the isolation oxide region during subsequent heating of the substrate. This places significant limitations on the processing steps used to fabricated the semiconductor device and, in particular, significantly limits the thermal processing of a device.